Pushrod question... Will it cause damage???

My Dad is done assembling and installing the motor and has begun working on all the accessories, and happened to mention to me that he is not really sure if he put the right length pushrods in, and that he'd know right away once the motor ran cause it would run like crap.
Now, my old man is not really a mechanic per se, but he's built some nice Mo's in his life that ran well. My concern is that maybe some damage could occur with the wrong size push rods???